Fix: Race condition between optimize and checkpoint causes infinity crash

[zpf121]

Close #3325

Actual behavior and How to reproduce it

Description

Running optimize and checkpoint simultaneously can cause the database to crash with an UnrecoverableError.

Error Log

[20:44:24.615] [1856579] [critical] UnrecoverableError: Invalid buffer type: ToMmap
   0# infinity::PrintStacktrace at /home/weilongma/zpf/infinity/src/common/utility/exception_impl.cpp:47
   1# infinity::UnrecoverableError at /home/weilongma/zpf/infinity/src/common/utility/exception_impl.cpp:81
   2# infinity::BufferObj::ToMmap at /home/weilongma/zpf/infinity/src/storage/buffer/buffer_obj_impl.cpp:386
   3# infinity::NewTxn::TryToMmap at /home/weilongma/zpf/infinity/src/storage/new_txn/new_txn_data_impl.cpp:2142
   4# infinity::NewTxn::CheckpointTable at /home/weilongma/zpf/infinity/src/storage/new_txn/new_txn_data_impl.cpp:1396
   5# infinity::NewTxn::CheckpointDB at /home/weilongma/zpf/infinity/src/storage/new_txn/new_txn_impl.cpp:1889
   6# infinity::NewTxn::Checkpoint at /home/weilongma/zpf/infinity/src/storage/new_txn/new_txn_impl.cpp:1802
   7# infinity::NewCheckpointTask::ExecuteWithNewTxn at /home/weilongma/zpf/infinity/src/storage/bg_task/bg_task_impl.cpp:54
   8# infinity::BGTaskProcessor::Process at /home/weilongma/zpf/infinity/src/storage/bg_task/background_process_impl.cpp:93

Reproduce

In TestMultipleIndexTypesImport, set:
pytest.param("test/data/config/restart_test/test_insert/3.toml", MultiIndexTypesGenerator, 5, 10, 120, marks=pytest.mark.slow)

uv run python3 tools/run_restart_test.py --infinity_path=cmake-build-release/src/infinity --marker=slow --test_case=test_multiple_index_types_import.py 2>&1 | tee /tmp/123456.log